This invention relates to ruthenium complexes bearing a chiral diphosphine and a chiral diamine and their use as catalysts for asymmetric hydrogenation processes.
A large and constantly growing number of catalysts and methodologies are at present available for the homogeneous asymmetric hydrogenation of functionalised ketones. Such ketones bear an auxiliary group that is positioned at the appropriate distance from the carbonyl group and which is capable of binding to the metal of the catalytically active species. This binding arrangement presumably allows chelation of a functionalised ketone to the metal center of the catalyst. The references to these catalysts and methodologies are comprehensively listed by R. Noyori in Asymmetric Catalysis in Organic Synthesis (John Wiley and Sons, New York, 1994) and by I. Ojima in Catalytic Asymmetric Synthesis (VCH, New York 1994).
Catalytic asymmetric reduction of unfunctionalised ketones presents a greater challenge. Unfunctionalised ketones are those lacking a secondary metal-binding group. EP-A-0718265 describes a method for producing alcohols from carbonyl compounds by hydrogenation in presence of a ruthenium catalyst, a base and a nitrogen-containing additive. When a ruthenium complex bearing a chiral diphosphine was used as catalyst in presence of a chiral diamine and a base, highly productive and enantioselective hydrogenation of aromatic ketones was achieved. Examples of chiral diphosphines examined were BINAP, Tol-BINAP, Xylyl-BINAP, H8BINAP and CHIRAPHOS Examples of useful chiral diamines were DPEN and DAIPEN. These respective compounds have the formulae 
It also has been disclosed by Noyori et al. (J. Am. Chem. Soc. 1995, 107, 2675 and 10417) that a diphosphine-ruthenium-diamine complex could be isolated and used as a pre-catalyst for the reduction of aromatic ketones. The use of such a preformed catalyst is advantageous for industrial applications. In particular, high productivity and high enantioselectivity were always associated with the use of the chiral biaryl-phosphines BINAP, Tol-BINAP and Xylyl-BINAP and the chiral diamines DPEN and DAIPEN, Xylyl-BINAP/DAIPEN being the optiumum combination (Angew. Chem. Int. Ed. Engl. 1998, 37, 1703 and J. Am. Chem. Soc. 1998, 120, 13529). For a review, see also Noyori, Angew. Chem. Int. Ed. 2001, 40, 40-73.
A wide array of diphosphines with a chiral backbone different from the biaryl backbone is known (see references listed by Noyori and Ojima in the references mentioned above). So far no chiral diphosphine, other than the BINAP-based ligands indicated above, has been reported to be useful in the efficient and highly enantioselective hydrogenation of unfunctionalised ketones according to the methodology described by Noyori.
PHANEPHOS (structure shown below), first described by Rossen et al. in J. Am. Chem. Soc. 1998, 119, 6207, is a chiral diphosphine based upon the rigid [2.2]-para-cyclophane backbone. In particular, PHANEPHOS-ruthenium complexes (described in Tetrahedron Lett. 1998, 39, 4441) have found an application, although limited, as catalysts for the asymmetric hydrogenation of xcex2-ketoesters (functionalised ketones). Good levels of activity and enantioselectivity were achieved only by working at high catalyst loading (0.4-0.08 mol%) and at low temperature (xe2x88x925xc2x0 C.). In addition, the PHANEPHOS-ruthenium complexes were reported to be of limited stability and to produce inconsistent results unless an external halide source (Bu4NI) was added. 
There is a fundamental structural difference between pseudo-ortho[2.2]para-cyclophane-diphosphines and the biaryl-diphosphines used by Noyori. This difference is highlighted by the contrasting results obtained in the ruthenium-catalysed hydrogenation of xcex2-ketoesters (see data reported in Tetrahedron Lett. 1998, 39, 4441 compared with those reported in Tetrahedron: Asymmetry Report Number 30, 1997, 20, 3327).
GB-A-2351735 (published Oct. 1, 2001, i.e. after the priority dates claimed herein) discloses the asymmetric hydrogenation of 1-(3,5-bis(trifluoromethyl)phenyl)ethan-1-one, using a ruthenium-phosphine catalyst and a chiral diamine. In the Examples, DIOP is used; in the description, PHANEPHOS is mentioned as one of several possible alternatives. The procedure involves forming the catalyst in situ, the amine being added last.
This invention is based on the unexpected discovery that the parent ligand PHANEPHOS and its derivatives form stable ligand-ruthenium-diamine complexes of general formula 1 
wherein each Ar is an aromatic or heteroaromatic group of up to 20 atoms;
X is halide or carboxylate; and
R1, R2, R3, R4 are independently hydrogen, aryl or alkyl, optionally linked or part of a ring.
The novel complexes are highly active and enantioselective catalysts for the asymmetric hydrogenation of unfunctionalised ketones, in the present of a catalytic amount of base.
It will be appreciated by those skilled in the art that, rather than use a preformed complex 1, equivalent catalysis may be achieved by forming the catalyst, e.g. the complex 1 or an active species that can be generated therefrom, in situ. This will usually be done by the reaction of the diamine with a ruthenium complex of the ligand in the presence of, or followed by the addition of, the base required for the hydrogenation.
Despite their structural difference, complexes 1 perform as well as, and in several cases better than, the BINAP-Ru complexes described by Noyori as catalysts for the hydrogenation of a wide range of unfunctionalised ketones.
As is evident from the Examples presented below, the PHANEPHOS backbone produces inherently more reactive and selective catalysts then the BINAP backbone. The influence of the Ar substituents on phosphorus is surprisingly less marked and allows for fine-tuning of the catalyst for a particular application Typically, in order to achieve high ( greater than 95% ee) enantioselectivity, the choice of BINAP-based catalysts is restricted to those prepared from Xylyl-BINAP and the costly diamine DAIPEN.
In addition, it has been found that the group X does not necessarily have to be a halide, as it transpires from all the examples so far published, but a carboxylate group can be used instead. A compound of general formula 1 where X=CF3COO has been shown to catalyse the asymmetric hydrogenation of unfunctionalised ketones, giving results comparable to complexes where X=Cl.
The novel ruthenium complex includes a diphosphine moiety that is a (R) or (S)-pseudo-ortho-bisphosphino-[2.2]-para-cyclophane where each phosphorus atom bears two additional aromatic groups. Ar is any aromatic group of up to 10 or 20 carbon atoms and is typically phenyl, optionally bearing one or more substituents. For many applications, the simplest ligand, where Ar=Ph, is applicable. In other applications, it is beneficial to use ligands in which Ar=phenyl substituted with one or more alkyl or alkoxy groups. Particular examples are Ar=3,5-dimethylphenyl, 4-methoxyphenyl and 4-methoxy-3,5-dimethylphenyl. In a preferred embodiment, Ar=3,5-dimethylphenyl (alternatively defined as xylyl) and the phosphine is referred to as 3,5-di-Me-PHANEPHOS.
The chiral diamine is preferably any 1,2-diamine with at least one stereogenic centre and R1, R2, R3, R4 are independently hydrogen or aromatic or alkyl groups, typically of up to 10 or 20 C atoms, optionally linked or part of a ring. Suitable diamines are of formulae 2 to 6 
or the opposite enantiomer thereof.
For example, the diamine is DPEN or DACH; both are readily available in either enantiomeric form, and as cheaper than DAIPEN.
X is a halogen atom or carboxylate. Suitable carboxylates are derived from a carboxylic acid of formula R5COOH, wherein R5 is an aromatic or alkyl group of up to 20 atoms, optionally bearing fluorine atoms. For example, X is Cl or CF3COO, and is preferably Cl.
This invention involves the synthesis of ruthenium complexes of general formula 1 and their use as catalysts for asymmetric hydrogenation of ketones in the presence of a base, according to the procedure already described by Noyori (EP-A-0718265; Angew. Chem. Int. Ed. Engl. 1998, 37, 1703; J. Am. Chem. Soc. 1998, 120, 13529). Examples of hydrogenation of acetophenone under different conditions are given below (see Tables 1 and 2). Examples (see Tables 3-5) are also given where a range of differently substituted aromatic ketones and xcex1,xcex2-unsaturated ketones are hydrogenated with high activity (typically, full conversion, 0.0033 mol% catalyst, 0.5-3 hours) and selectivity (typically  greater than 97% ee). In such hydrogenations, it is preferred that a particular enantiomer of the disphosphine ligand is matched with the correct enantiomer of the diamine. This is evident from entries 4-7 in Table 2.
Complexes 1 of the present invention are prepared by combining the diphosphine, the diamine and an appropriate ruthenium precursor in a solvent. According to the published literature, diphosphine-ruthenium-diamine complexes 1 may be prepared from [ruthenium-benzene-Cl2]2 in DMP, followed by reaction with the diamine in any suitable solvent, e.g. DMF or dichloromethane. An alternative procedure for the synthesis of complexes of formula 1 (for example where X=Cl) involves initial synthesis of the cationic intermediate [diphosphine-ruthenium-benzene-Cl]Cl which subsequently is reacted with the diamine in DMF (Scheme 1). Such complexes are solids suitable for storage under an inert atmosphere, by that can be handled in air.
Complexes of general formula 1, where X=CF3COO, may be prepared according to a modified procedure, which involves reacting the complex [3,5-di-Me-PHANEPHOS-Ruxe2x80x94(OOCCF3)2]2 (see K. Rossen et al. in Tetrahedron Lett. 1998, 39, 4441, for preparation of [PHANEPHOS-Ruxe2x80x94(OOCCF3)2]2) with a diamine in DCM/EtOH (Scheme 2). An example is given where the diamine is DPEN. An example of hydrogenation of acetophenone using this complex is given in Table 2 (final entry). 
A catalyst of the invention is particularly suitable for the stereoselective hydrogenation of a ketone. For example, the ketone has the formula R6xe2x80x94COxe2x80x94R7, wherein R6 is an aromatic group and R7 is an alkyl group or the formula R8xe2x80x94COxe2x80x94R7, wherein R8 is alkenyl and R7 is an alkyl group. Such hydrogenation requires a base additive. Typically, the base is an alkali metal alkoxide or hydroxide and is preferably potassium tert-butoxide. At least one molar equivalent of the base relative to the catalyst is used, and typically 10 or 20 to 200 molar equivalents are used. Such hydrogenation reactions may be carried out by procedures that are known to those skilled in the art.
The following Examples illustrate the invention. More specifically:
Examples 1 to 3 illustrate some specific preparations of the ruthenium pre-catalysts derived from 3,5-di-Me-PHANEPHOS.
Examples 4 and 5 illustrate two general procedures for the synthesis of ruthenium pre-catalysts derived from PHANEPHOS and its derivatives. Various procedures are possible, producing equally effective pre-catalysts.
In Example 6 (Table 2), the hydrogenation of acetophenone is used to show that a strong matching/mismatching effect is displayed by a number of different PHANEPHOS derivatives and diamine ligands, the (R)-ligand-(S,S)-diamine diastereoisomer (or the opposite enantiomeric pair) being the most effective combination. In addition, it is apparent that the best results are obtained when the PHANEPHOS derivatives are matched with the diamines DPEN and DACH.
Example 6 (Table 1) compares the results obtained with the catalysts based on the parent ligands PHANEPHOS and BINAP. Rate and selectivity obtainable with the PHANEPHOS catalysts clearly indicate that the effectiveness of the catalyst depends mainly on the structure and chirality of the backbone rather than on the substituents at the phosphorus atom.
Examples 7 to 9 show that the hydrogenation of acetophenone with [3,5-di-Me-PHANEPHOS-Ruxe2x80x94Cl2-DPEN] can be easily and conveniently scaled up, to hydrogenate up to 100 g of substrate at very economical catalyst loadings.
Examples 10 and 11 demonstrate the scope of the hydrogenation catalysed by [3,5-di-Me-PHANEPHOS-Ruxe2x80x94Cl2-DPEN]: a number of aromatic, hetero-aromatic and xcex1,xcex2-unsaturated ketones are smoothly hydrogenated under mild conditions.
Examples 12 and 13 show the advantages obtainable with PHANEPHOS-ruthenium catalysts in the hydrogenation of a specific, sterically hindered substrate, i.e. 2-MeO-acetophenone. Rates, selectivity and catalyst loadings compare very favourably with the results reported in the literature for the best BINAP-ruthenium catalysts.
In Examples 6-13, hydrogenation reactions in which the catalyst contains a (R)-PHANEPHOS ligand give as major product the (R) alcohol. Likewise, the (S) ligand gives primarily the (S) alcohol.